1. Field of the Invention
This invention relates to an overcurrent prevention circuit, and more particularly to a circuit which prevents overcurrent through a signal line between two devices which operate with power supplies which supply power supply voltages different from each other.
2. Description of the Related Art
In a system which includes two devices between which a signal is communicated, it is desirable to provide an element for removing noise such as a clamp diode on the power supply side of an input terminal of a receiving end side of one of the devices. Conventionally, however, where the system described above is of the type wherein different power supplies are used for the two devices, such a noise removing element as described above is not provided because, if the noise removing element is provided, then overcurrent is likely to flow therethrough and between the two devices and cause a breakdown of the devices and the noise removing element.
FIG. 4 shows a transmission-reception system including a transmission device and a reception device which operate with different power supplies. Referring to FIG. 4, in the transmission-reception system shown, a transmission device 41 operates with a power supply VCC1 while a reception device 42 operates with a power supply VCC2 different from the power supply VCC1. The transmission device 41 and the reception device 42 are connected to each other by a signal line. In this instance, if it is assumed that a clamp diode 40 is inserted in the signal line as seen in FIG. 4, then when the power supply VCC1 is in an on-state and the power supply VCC2 is in an off-state, overcurrent flows through the clamp diode 40. Therefore, in the conventional transmission-reception system, actually the clamp diode 40 is not inserted.
The transmission-reception system described above is disadvantageous in that it cannot remove noise because a clamp diode which serves as a noise removing element is not inserted in the signal line between the transmission device and the reception device. Meanwhile, an apparatus for preventing production of noise in a bus is disclosed in Japanese Patent Laid-Open Application No. Heisei 1-163812. However, the document just mentioned is silent on the prevention of a breakdown of a noise removing element and therefore cannot solve the disadvantage of the conventional transmission-reception system described above.